boiler water wall tube leakage

[hemnish]

We have two 390 t/hr CFBC boilers supplying steam at 130 ksc and 540 deg C.The boiler has four FBHE's.In one unit we are facing repeated problem of water wall tube leakage in FBHE 2(super heater 2B).Tube leakage occured four times around spiess valve area,two times in straight tube.Boroscopic inspection for checking tube choking did not reveal any result earlier. Flushing of the tubes at high pressure did not yield any result either.
But in the latest failure it is found that one tube is in badly choked condition,the chemical analysis of which has shown that the deposit is 98 % ferrous related (oxides of iron).
My questions are : (1)What could be reason for such deposit after two years of operation that too after acid cleaning and passivation?
(2)How tgo ensure other tubes are not effected at present?
(3)what is the corrective action we can take to aviod such tube failure in future?

Thanking you.

 

[pksinha]

Look at the water chemistry over a period of time vs recommeded chemistry by boiler manufacturer.

 

[LuizSouza]

Yes, Pksinha is correct,
Some use to forgett this detail

 

[pablo02]

Follow up with the suggestions mentioned -- if this is occurring in a supeheater section, also look for either carry-over from the drum or material (or chemicals) coming though any SH attemperating sprays...

 

[PeteBennett]

Would also check if you are returning condensate back to the feed tank, do an analysis on this?

Are you blowing your boiler down often enough i.e Bottom, Side and TDS?

Is your feed water being oxygenated from some source?

If you have a water preheat system from say, excess flash, whatever, is there cross contamination from the heat exchanger?

 

[97750]

in this power plant boilers made in italia
max.press. is a 220 bar
a big problem in p.p is tupe rapther
i want know why?

 

[pablo02]

97750: tube ruptures occur for a number of reasons; your first step is identifying where the rupture occurs (water wall tubes, near burners or fire-ball, radiant SH, convection SH, etc.) and the type of rupture (pin hole, corrosion, thick wall failure, thin wall fish mouth, etc...) then you try analyze what caused the failure (chemical upsets, long-term overheat, short-term overheart, wrong materials, flow restrictions, stress corrosion, etc)

many times, you'll need the assistance of your chemical supplier or lab, the boiler OEM, or metallurgist to help you determine the cause -- or find someone who has dealt with boilers for a period of time and give them sufficient detail (tube samples to analyze / look at)and they might be able to point out the problems...

can not be determined without a lot of information right in front of one...

 

[davefitz]

I would recommend a metallurgical analysis of the failed tube sections. The analysis will show the failure mode, and determine the characteristics of the tube. There are so many possible causes of tube failure taht there is no hope for finding the cause without such an analysis.

You have indicated one of the failed tubes had shown signs of "badly choking"- I assume that means pluggage of the tube inlet section with debris. If so, then you need to use a boroscope and investigate all the inlet headers to determine the amount and kind of debris.

 

[pablo02]

davefitz -- I tend to agree with you to a point -- however, someone who is familiar with their boilers and has the experience in chemistry and failure modes can usually determine the type of failure as well as the corrective action needed without paying the expense for a metallurgist; however, in this day and age, that luxury is probably a thing of the past...

[in over 50 years at my former company, we used a metallugist twice and he only confirmed our own analysis]...

 

[yogibear1]

No suggestions, just wanting to know the meaning of terms used: what kind of boiler is CFBC - maybe circulating fluidized bed?? What are FBHE's?? What is the spiess valve area? Thank you for the information.

 

[cbjohnk]

I am a little late coming to this thread, but there is no indication that the problem has been resolved.

What exactly do the failures look like? Do you have any pictures? Are the failures in refractory covered areas? If so, was the refractory worn away before the failure? If not, is there wear from the outside (I know that is a dumb question, because that should be pretty obvious, but...)?

One problem that can occur with a tube rupture in the furnace of a fluidized bed, especially in the area near the "spiess" valve, is that during the cooldown alot of ash can get into the tubes and block that tube or others in the vicinity. Sometimes this blockage is not obvious because the blockage can be washed away when the leak occurs. You should look in the headers for debris (if you haven't already).

If you have anymore information that might help determine what your problem is let me know.

Regards,

Carl Johnk